Conventionally, multi-tone original images, or in other words, continuous-tone original images, have been screened into halftone dots and recorded on recording media. In screening original images, amplitude modulated (AM) screening or frequency modulated (FM) screening is used. In AM screening, gradations are expressed by changing the size of halftone dots without changing the number of halftone dots. In FM screening, on the other hand, gradations are expressed by changing the distribution density of irregularly arranged microdots, which are minimum pixel units.
Inkjet image recording apparatuses or the like often perform screening through FM screening. In the case of screening a multi-tone original image into halftone dots through FM screening, generally a threshold matrix in which a plurality of elements are arranged in both row and column directions and are each given a threshold value is generated and prepared in advance, and this threshold matrix is compared with the original image.
A technique for generating a threshold matrix is disclosed in, for example, Japanese Patent Application Laid-Open Nos. 2008-294702 (Document 1) and 2009-61727 (Document 2). In the threshold matrix generation method of Document 1, a plurality of linear areas extending in a direction inclined relative to row and column directions are preset in a matrix area. Then, the linear areas are alternately allocated to a first element group and a second element group. In each of the element groups, the order in which dots are lit is determined by repeating processing for specifying one of the elements and then specifying an element that is most distant from the specified element. The threshold value of each element is determined in accordance with this order.
On the other hand, Document 2 discloses a technique for recording an image at 2× (double) speed, using an image reduced by half in a width direction corresponding to the row direction of the pixel array. In 2×-speed writing, an image is recorded while suppressing degradation in image quality by enlarging the reduced image in the width direction and writing dots in a checkerboard pattern in the row and column directions with respect to writing positions in normal writing.
Incidentally, when inkjet image recording apparatuses or the like perform high-speed image recording, landing positions of fine ink droplets ejected from an inkjet head onto an object on which an image is to be recorded (hereinafter, a “recording object”) may differ from desired positions. If the landing positions of ink are disordered when recording an image that has been screened into halftone dots through FM screening, the density of dots on a recording object may be biased, and graininess may increase (i.e., graininess may deteriorate) in, for example, a highlight area (i.e., a bright portion) where the image has low gradation values. On the other hand, screening performed through AM screening can reduce graininess, but there is the possibility of moires occurring as a result of interference between the periodicity of the arrangement of regularly arranged halftone dots and the periodicity of the arrangement of nozzles in an inkjet head.